green watery poop

My 5 year open cat has green watery stool, threw up last night and he has been very mellow. He is an indoor cat and his food hasn't changed as well as his eating habits. Any ideas?

Something in the food may have changed, or his gut bacteria got out of balance from stress (cats may stress at stuff we don't understand). He may have developed IBD - the green diarrhea means everything is zipping through him too quickly, so he's not absorbing needed nutrition. A vet visit is definitely in order! Bring a stool sample.

This could be SO many different things.